Laser cutting provides a precise method for machining intricate and small patterns in pieces of metal, polymer, and other materials. A particular application of interest is the machining of metallic tubes for use as intravascular stents. The stents are designed to be inserted into the vasculature, including the coronary arteries. Stents usually include open spaces in the tube walls to provide vessel support with minimal intimal contact. Stents can be formed in patterns comprising mostly open space, such as lattice works or helical spring patterns. Such patterns require being able to burn through one wall of a tube without burning through the opposite wall.
Laser cutting of a tube proceeds by focusing a laser beam on a targeted spot of the tube. The spot is melted and preferably substantially vaporized by the laser beam. Once the laser beam burns through the wall, the beam strikes the opposite wall within the tube, and can begin to heat the opposite wall from the inside, burning through the opposite wall. This problem is termed "burn through". The melting and vaporization can also leave behind dross, including metal oxides, impurities and fragments of the metal being burned. The dross can fall into the kerf being burned and adhere to the interior of the tube being burned. Both dross adherence and burn through are undesirable. The dross requires removal from the product and burn through can require discarding the product.
One method used to deal with dross adherence and burn through is flushing or sluicing liquid through the tubular workpiece being cut. The fluid is feed in one end of the tube, exiting through both the opposite end of the tube and through the opening burned in the tube wall by the laser. Water is one fluid used. Water within the tube can flush away dross which could be inside or outside of the laser cut opening. The water can also serve to attenuate the laser beam striking the opposite wall. The presence of water within the tube being cut may not completely block the laser beam, allowing the inside wall to be heated. The use of water requires additional equipment for handling the water including supply, catch basins, disposal, and often filtration and reuse. The water must be connected to the tube being cut, which is preferably movable under the laser beam. The tube is preferably rotatable while connected to the water supply. Optimally, the tube can be rapidly mounted in the laser cutting device. The requirement for water complicates the rotation of the tube being cut and can lengthen the time required to mount the tube in the laser cutting device. What is desirable and has not heretofore been provided is a device for preventing burn through and dross accumulation in tubes being cut by laser that does not require the use of a liquid flowing within the tube being cut.